Recently, in order to maximize the performance and communication capacity of a wireless communication system, a Multiple Input Multiple Output (hereinafter referred to as ‘MIMO’) system has been in the spotlight. The MIMO technique, unlike the existing technique using one transmit antenna and one receive antenna, adopts multiple transmit antennas and multiple receive antennas in order to improve the transmission efficiency of transmission and reception data. The MIMO system is also called a multi-antenna system. The MIMO technique is an application of a technique for gathering short pieces of data received from several antennas at one place and completing them in a single message, without depending on a single antenna path in order to receive the entire single message. Consequently, the data transmission rate can be enhanced in a specific range or the system range for a specific data transmission rate can be enhanced.
In the MIMO system, a user equipment feeds back channel state information (CSI) to a base station. Here, the channel state information can be fed back through a variety of information. For example, the channel state information can be fed back through a Precoding Matrix Index (PMI), a Channel Quality Indicator (CQI), a Rank Indicator (RI) or the like. The user equipment performs channel measurement based on a reference signal transmitted by the base station and feeds back a preference PMI and a preference RI to the base station on the basis of a result of the channel measurement. The preference PMI and RI can be a PMI and RI which is determined to produce the highest transmission rate in the case in which it is used by the base station under a specific channel state. The CQI indicates a Modulation and Coding Scheme (MCS) to guarantee an adequate ratio of packet errors in a PMI and RI which is fed back by the user equipment. The base station may use the channel state information, fed back by the user equipment, in scheduling.
In a conventional 3GPP Long Term Evolution (LTE) system, a user equipment assumes a single cell single user MIMO method in a process of calculating a CQI/PMI/RI. In other words, assuming that a serving base station allocates the entire transmission power to the user equipment and does not operate in conjunction with the base stations of neighbor cells in accordance with a cooperative multi-points transmission and reception (CoMP) method, the user equipment calculates the CQI/PMI/RI. The single cell single user MIMO method is a basic operation method in the MIMO technique of an LTE system. The single cell single user MIMO technique is hereinafter abbreviated to a normal operation.
A base station can be operated in a MU-MIMO mode or a CoMP mode as well as in the normal operation. A method of a user equipment feeding back channel state information, regulated in the existing LTE system, is difficult to apply to the MU-MIMO mode or the CoMP mode without change. This is because as described above, the user equipment assumes the normal operation in the process of calculating the channel state information.
Accordingly, in order to support a variety of transmission modes (e.g., the MU-MIMO mode and the CoMP mode) between a base station and a user equipment, there is a need for a CSI feedback method different from a CQI/PMI/RI feedback method defined in the existing LTE system.